1. i, j, k: Extremely common in loop constructs, especially in languages like C, C++, Java, and others that follow C-style syntax. The practice of using these letters as loop counters is deeply ingrained in the programming community.

2. n, m: Widely used for denoting sizes or dimensions, particularly in arrays or sequences. These variables are commonly used across various programming languages.

3. digit: Less common than others, but used when dealing specifically with individual digits of a number. Its usage is more domain-specific, often seen in algorithms and mathematical operations.

4. arr: Frequently used to signify arrays or lists. Its usage is prominent in languages like Python, where developers often use descriptive variable names to enhance code readability.

5. temp: Commonly used to represent temporary variables in various programming contexts. Its usage is widespread, especially when a short-lived variable is needed for intermediate calculations.

6. count: Regularly used for keeping track of occurrences or counting elements. Its usage is prevalent in loops, conditional statements, and scenarios where counting is required.

7. index: Commonly used to represent the position of an element in a sequence, often seen in array-related operations. Widely used in various programming languages.

8. value: Often used as a generic name for a variable that holds a specific value, especially in scenarios where the nature of the value is not explicitly defined.

9. result: Typically used to store the outcome or result of a computation or function. It adds clarity when a variable is specifically designated to hold the final result.

10. input: Reserved for variables that store user inputs or external data, enhancing code readability when dealing with data entered by users or external sources.

11. output: Similar to 'result,' this variable is often used to store the final output of a function or operation, helping distinguish it from intermediate values.

12. flag: Used to represent a boolean variable that signals a particular condition or state. It's commonly employed in control flow structures to make code more readable.

13. sum: Frequently used to accumulate values, especially in loops or mathematical operations where the sum of multiple elements is needed.

14. avg: Represents the average of a set of values. It's commonly used in statistical or mathematical contexts.

15. max, min: Used to denote the maximum and minimum values, respectively. These are common in algorithms that involve finding extremes in a dataset.

16. buffer: Often used in scenarios involving data storage or manipulation, especially when dealing with arrays or streams of data.

17. ptr (pointer): Commonly used when working with memory addresses or dynamic memory allocation, particularly in languages like C and C++.